Diaporthe phaseolorum var. caulivora isolated from soybean plants infected with stem canker produces a toxin that caused symptoms characteristic of stem canker when introduced into the plant. The amount of toxin and the degree of symptoms were linearly related. Four fungal isolates produced the toxin in culture in significantly different amounts, when measured in terms of the degree of symptoms and the dilution end point. The amount of toxin produced by the isolates correlated with the length of cankers produced by the fungus upon inoculation. Of the 12 plant species evaluated, only soybean and lima bean were sensitive to the toxin and also susceptible to the fungus. The purified toxin produced symptoms very similar to those of stem canker, and some of its chemical properties are similar to those of a phytotoxin suggested to have a role in pine wilt caused by Phomopsis sp. The D. p. caulivora toxin appears to play a role in stem canker of soybean.